Hi Jon, I tried this, could not find a way of tightening it.
My solution, get some greese and lubricate the top strip and also the lock where it clips in when the roof is down. This sorted mine for a while. Looks like the lock is a sealed unit on the joints so I couldnt work out how to tighten it.
To be honest summer is round the corner, so you roof will be down soon mate.
Have you also noticed on the A4 Cab the rattle / squeek about the sun visor ??